[StackSafety] Add "Must Live" logic

Summary:
Extend StackLifetime with option to calculate liveliness
where alloca is only considered alive on basic block entry
if all non-dead predecessors had it alive at terminators.

Depends on D82043.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82124